Particular reference will be made below to extrusion presses fed with billets, usually aluminium billets, although it is understood that the invention can be applied to other types of presses also.
The extrusion presses for billets known at present comprise a fixed cylinder on a fixed base, inside of which cylinder a ram or main piston is movable. A movable cross member is integral with the ram, and usually guided along the base, and carries a stem extending along the extrusion axis. On the same base or another base a fixed cross member is mounted, which carries a die slide, and in the die slide various dies can be interchanged.
A so-called container having a through chamber for the billet, is movable on the base along the extrusion axis between a working position, in which it is in a sealed or tight manner against a die contained in the die slide, and a rest position, in which it is at a distance from the box which is sufficient for the passage of a shears blade moving transversely to the extrusion axis. The container is moved by cylinders located on the fixed cross member.
The presses so briefly described operate in the following manner. With the container positioned in an airtight or sealed manner against the die slide complete with a die, a billet at a suitable temperature, which has been carried onto the extrusion axis in any way whatsoever upstream of the container, is first introduced into the container and then pressed against and through the die by means of the stem; the stem moves back at the end of its run and the container is moved away from the die by the cylinders of the container, in such a way that the shears can intervene for separating the produced profile or section from the remaining part of the billet, known as the butt or scrap.
The process for preparing a press for extruding a certain quantity of aluminium alloy often leads to air being trapped in the bore of the container, before the alloy being subjected to extrusion pressure. The air is pressurized by the extrusion process and causes serious damage to the finish of the product, in the form of blisters, often accompanied by characteristic noises made by the air when it expands in the atmosphere after passing through orifices.
A standard cycle for an extrusion process, with the aim of eliminating the problem of trapped air, is called a degassing or "burp" cycle, or pre-pressing cycle, etc. According to this cycle, the aluminium alloy is compacted in the container (positioned against the die) at a pressure equal to or lower than half of the pressure required for extrusion. Such operation is called upsetting of the billet. Then the members which usually are on load during extrusion are released, that is to say, the ram, the stem, the container and the die; in other words, these members are physically moved so as to break the sealed or airtight engagement around the die, allowing air to escape. The cycle which includes degassing is then completed by closing the container once more against the die until a sealed or airtight engagement is obtained, while the main ram reassumes the position for the beginning of the extrusion.
The above described process requires a certain time lapse (5-6 seconds) for inverting the movement of the members; this reduces the productivity of the press.
A method for avoiding the loss of time inherent in the degassing cycle is the object of European Patent application No. 90305003. 7; this method comprises removing air from the container by connecting a vacuum-creating system to the container itself. An air suction or vacuum line is made in the stem of the press, the stem having a pressure head or pad movable between a position in which it frees an aperture of the vacuum line and a position where it closes it. The equipment, however, for realizing such method is very expensive.